Did you have an iPhone prior to you getting the S5? If yes then you need to disassociate your phone number from iMessenger.
How long ago did you port? It may take up to 2 weeks for everything works 100%(this doesn't matter which carrier) but generally should be done within 48 hours.
just ported today. Here's a question though. Ironically, I had the same issue on my blackberry the last week prior to porting....anything to that or just ironic?
Might be just ironic. Give it another 24 hours after than then it may require a call to customer care.